In general, a plunger rod is press-fitted into a gasket in advance and a normal syringe is obtained by inserting the gasket into a barrel. In contrast, a medicinal solution-filled syringe, that is, a prefilled syringe is normally obtained by filling medicinal solution in the barrel, plugging with the gasket, and joining the plunger rod with the gasket. A method of joining the plunger rod with the syringe plugged with the gasket as a post-process is employed as a method of obtaining the prefilled syringe. The reason why this method is employed is that if the plunger rod is attached to the gasket in advance, it is difficult to carry out a gasket plugging process for sealing.
As the method of joining the gasket and the plunger rod of a prefilled syringe as described above, screw-engagement between a female thread of a gasket formed of a rubber elastic material such as butyl rubber or elastomer and a male thread of a plunger rod formed of plastic such as polypropylene is widely employed. It is because, if the plunger rod is press-fitted into the gasket as in the case of a normal syringe, the medicinal solution might be leaked from the gasket or a distal end nozzle by the pressure thereof.
However, the joint between the gasket and the plunger rod by direct screw-engagement in the related art may be loosened due to variation of dimensional tolerance at the time of manufacture or variation in fastening torque of an assembling machine. In such a case, when the prefilled syringe is transported on a motor vehicle for example, the plunger rod is rotated and hence released from screw-engagement due to vibrations or the like applied to the prefilled syringe, so the plunger rod is in danger of coming off. The plunger rod might also come off during manufacture as well.
In order to prevent the plunger rod from coming off, the joint between the gasket and the plunger rod has been studied, and improved joint mechanisms have been proposed. For example, there is proposed a plunger including a plunger rod having a top surface provided with a male thread and a gasket provided with a female thread to be screw-engaged with the male thread, in which the top surface of the plunger rod is provided with a projection projecting toward the distal end thereof and the gasket is provided with a recess to be fitted with the projection, so the gasket and the plunger screw-engaged to each other are hardly disconnected owing to the fitting between the projection and the recess (Patent Document 1).
There is also proposed a syringe including a plunger having a screw-fit portion formed inside a gasket which is to be screw-fitted with a flange formed at the distal end portion of a plunger rod and a space formed on the side of the distal end of the screw-fit portion, in which when the flange formed on the distal end portion of the plunger rod is advanced to the space beyond the screw-fit portion, the flange is engaged with the space in a loosely fitted state, so an external force exerted on the plunger rod is prevented from being transmitted directly to the gasket (Patent Document 2).    Patent Document 1: JP-A-2005-80957    Patent Document 2: JP-A-2002-272843